The present invention relates to a metal halide high intensity electric discharge lamp, and more particularly a metal halide lamp having a starting electrode for each of the lamp discharge electrodes, for improving lamp maintenance.
The use of auxiliary starting electrodes in electric discharge lamps is known. For example, a high pressure mercury vapor discharge lamp having a pair of auxiliary starting electrodes is disclosed in the book High-Pressure Mercury Vapor Lamps and Their Applications, edited by W. Elenbaas, 1965 at page 114. In this lamp the applied voltage is applied not only across the pair of main discharge electrodes, but across each main discharge electrode-auxiliary electrode pair. The applied voltage causes a glow discharge and local ionization in the gap between a main discharge electrode and its adjacent auxiliary electrode, and the electrons thus produced are accelerated between the main discharge electrodes until an arc discharge is fully developed.
Generally, electric discharge lamps use only a single auxiliary electrode. Lamp ballasts have become sufficiently reliable to reliably start discharge lamps, the second auxiliary electrode was dispensed with, and today only a single auxiliary electrode is used. Thus, the standard work on discharge lamp technology, Electric Discharge Lamps by John Waymouth, 1971, teaches the use of a single auxiliary electrode in both high pressure mercury vapor discharge lamps and metal halide discharge lamps.
The use of two starting electrodes in the prior art was solely for the purpose of facilitating starting. Both Austrian Patent No. 242,239 issued Sept. 10, 1965 and Swiss Patent No. 481,550 issued Dec. 31, 1969 disclose discharge lamps having a pair of starting electrodes. In both patents the disclosed improvements relate to lamp starting, and there is no suggestion that the pair of starting electrodes had any other purpose or effect than to improve starting. Moreover, neither of these patents teach the use of two starting electrodes in a metal halide lamp.
U.S. Pat. No. 3,275,397 issued Sept. 27, 1966 discloses a method for processing discharge lamp arc tube assemblies. An arc tube assembly comprises a discharge tube having a pair of discharge electrodes and a pair of auxiliary electrodes. The discharge tube is filled with an inert starting gas, mercury and a metal halide additive. According to the disclosed method an electrical discharge is first developed between the discharge electrodes for driving off occluded gases. Next, an electric discharge is developed between the refractory metal starting electrodes for gettering gaseous impurities within the discharge tube. Thereafter, the arc tube assembly is installed in a finished lamp. There is no suggestion to install the processed arc tube assembly in a lamp so that both starting electrodes are operative.
The problem of improving the lumen maintenance of metal halide lamps is a continuing one. The highly reactive atmosphere within the discharge tube of a metal halide lamp inevitably leads to the deterioration of the discharge electrodes, and to the deposition of material on the discharge envelope walls which darken it. Additionally, these lamps contain sodium as a principal light emitting material, and sodium loss during lamp operation contributes to deterioration in lumen maintenance.